An Overview
of Ozone Depletion in the Stratosphereby scientist and author
Rod Jenkins

FOREWORD: This is a tutorial and teacher's guide for classroom
studies in both high schools and colleges. It provides a summary in easy-to-understand
terms of the current scientific understanding of ozone depletion (i.e., ozone destruction
in the stratosphere or 'ozone hole'). It is intended to provide a bridge between
the scientific community and students on the key findings and the extent of our current
understanding. Graphics are used extensively to help convey the concepts. A comprehensive
glossary of terms is provided at the end.

I. The Ozone Layer

All life on earth exists in a very thin layer called the biosphere.
This consists of a layer of soil and water and atmosphere that is about 35 miles
thick. To put this in perspective, the thickness of the biosphere for planet earth
is comparable to the thickness of the skin on an apple.

Within the biosphere, above the highest mountain, lies a layer
of the atmosphere called the stratosphere. This is also known as the "ozone
layer" because of the critical role that ozone plays there in protecting the
earth from ultraviolet radiation.

The stratosphere, or ozone layer, is located approximately
6 to 30 miles above the earthís surface. In contrast to the lower atmosphere where
turbulence and vertical mixing occur, the stratosphere is relatively quiescent. As
a consequence, it is particularly susceptible to contamination because pollutants
introduced there tend to remain for long periods of time (from several years to decades).
Figure 1 shows the location of the stratosphere relative to the other four principal
layers of our atmosphere. Most of our atmosphereís gases are contained in the troposphere
and stratosphere, as shown in Figure 2.

Figure 1The Layers of Our Atmosphere

A. What Is Ozone?

The molecules of oxygen that we need to breath are composed
of two atoms of oxygen (diatomic oxygen). Ozone is a molecule that contains three
atoms of oxygen (triatomic oxygen). Ozone is concentrated in the stratosphere as
shown in Figure 3. However, the actual amount of ozone is very small. Scientists
measure the ozone directly above us in Dobson units. A Dobson unit can be best understood
as a 0.01 mm layer of ozone if all the ozone where concentrated at ground level (at
0û C). A typical amount of ozone in the stratosphere above the United States
is about 350 Dobson units which is equivalent to layer of ozone only 3.5 mm thick
at ground level.

Figure 2
Atmospheric Concentrations vs. Altitude

Figure 3
Ozone Concentration vs. Altitude

B. Why Is Ozone Important?

In the stratosphere, ozone absorbs virtually all of the solar
ultraviolet (UV) radiation with wavelengths of less than 290 nanometers (nm) and
most of it in the biologically harmful wavelength region of 290 to 320 nm (which
is called UVB). This prevents the radiation from reaching the surface of the Earth
in quantities which could adversely affect the lives of human beings, plants, and
animals. The UV radiation that is not absorbed are the same rays of the sun that
cause us to sunburn and otherwise damage our skin. This UV absorption is mostly responsible
for the temperature inversion (temperature increase with increasing altitude) that
characterizes the upper stratosphere and produces its quiescent nature. Ozone also
absorbs strongly in the infrared part of the spectrum, and this absorption plays
a part in maintaining the heat balance of the globe.

There is a great difference between the beneficial effects
of ozone in the stratosphere and its effects in the lower troposphere. In the lower
troposphere, ozone is very reactive and harmful to plants, animals and even structures.
For this reason, great efforts are taken in our communities to try to reduce ozone
pollution at ground level.

C. The Natural Cycle of Ozone Creation and Destruction

Stratospheric ozone has played a major role in the evolution
of life on earth. Over a very long period of time, oxygen given off by primitive
plant life (e.g., ocean phytoplankton) began to drift up into the atmosphere. As
it did, UV radiation from the sun would strike the oxygen molecule and break it into
two oxygen atoms. Some of these single oxygen atoms would combine with other oxygen
molecules to form ozone. The ozone molecules were much better than oxygen at absorbing
the harmful UV radiation. After millions of years, enough ozone was produced in the
stratosphere that life as we know it could survive and prosper under the heat and
radiation of the Sun.

Eventually, a natural balance between ozone production (as
described above) and ozone destruction was reached. See Figure 5. Natural ozone destruction
occurs as follows: When an ozone molecule absorbs UV radiation, the ozone molecule
breaks apart into diatomic oxygen and a single oxygen atom. Further ozone destruction
can occur when the free oxygen atom reacts with another ozone molecule to form two
molecules of diatomic oxygen.

Because of the dynamic nature of this balance, the amount of
ozone in the stratosphere isn't constant, but varies from place to place, month to
month, and year to year. Variations on time-scales of up to 11 years have been observed,
correlating with the solar cycle. Figure 6 shows the long-term annual variation in
total ozone in the Northern Hemisphere between 1933 and 1970 to be in the range of
± 5 percent (based on measurements at Aroza, Switzerland). Natural year to
year variations in the total ozone column can be as much as 1 percent, while day-to-day
changes can be greater than 10 percent.

Figure 6Long-Term Annual Variation in Total Ozone in the Northern Hemisphere

D. Recent Observations of Ozone Depletion

1. Antarctic Losses

In 1985, scientists from the British Antarctic Survey reported
that the ozone layer over Antarctica had shrunk substantially each September and
October since the late 1970s, which corresponds to the start of the Southern Hemisphere's
spring season. The drop in ozone levels in the stratosphere was so dramatic that
at first the scientists thought their instruments were faulty. Replacement instruments
were flown out and the measurements were repeated before the ozone depletion was
accepted as genuine. The decrease in the concentration of ozone over Antarctica is
shown in Figure 7. This phenomenon has come to be known as the Antarctic "ozone
hole." In 1994, the "hole" showed an ozone loss of about 65% and its
edges reached beyond the Antarctic continent to the tip of South America. Figures
8 and 9graphically show the growth in the size of the ozone hole from 1979
to 1994.

2. Global Losses

Research has shown that ozone depletion occurs over the latitudes
that include North America, Europe, Asia, and much of Africa, Australia, and South
America. Thus, ozone depletion is a global issue and not just a problem at the South
Pole.

In 1988, an exhaustive review of NASA satellite data concluded
that, averaged over the globe, ozone had decreased about 2.5 percent between 1969
and 1986. A 1991 NASA research effort revealed that the magnitude of ozone loss was
bigger, that the spatial extent was larger, and that the ozone depletion was persisting
for a greater part of the year than had been previously recognized (Zurer, 1993).
Ozone destruction over the Northern hemisphere's mid latitudes including highly populated
region such as the U.S. and Europe was two to three times as great as the scientists
had previously calculated.

In 1993, data from the Total Ozone Mapping Spectrometer (TOMS)
onboard the Nimbus 7 satellite showed that Global ozone levels for the winter of
1992 through spring of 1993 were 2-3 percent lower than in any previous year for
these months and 4 percent lower than normal. Ozone levels for the northern mid-latitudes
were about 10 percent lower than historical averages for this time of year and continued
at low levels into the early summer.

Over the U.S., ozone levels have fallen 5-10%, depending on
the season.

Further evidence that something has recently upset the ozone
balance in the northern hemisphere was provided by the city of Aroza, Switzerland.
Aroza has been making and keeping records of total ozone since 1926, longer than
any other location. Figure 10 shows that the level of ozone over Aroza barely changed
(a 0.1 percent increase per decade) between 1926 and 1973, but between 1973 and 1993,
this changed to an average 2.9 percent decrease per decade. The year to year changes
in ozone levels over the northern hemisphere are graphically shown in Figure 11.

Figure 10
Ozone at Aroza, Switzerland, 1926 - 1993

Figure 11
Total Ozone Averages, 1979 - 1994
Northern Hemisphere

E. Manís Contribution to Ozone Depletion

Only recently has manís activities begun to impact the ozone
layer, upsetting the natural balance of ozone creation and destruction processes.
For over 50 years, chlorofluorocarbons (CFCs) were thought of as miracle substances.
They are stable, nonflammable, low in toxicity, and inexpensive to produce. Over
time, CFCs found uses as refrigerants, solvents, foam blowing agents, and in other
smaller applications. Other chlorine-containing compounds include methyl chloroform,
a solvent, and carbon tetrachloride, an industrial chemical. Halons, extremely effective
fire extinguishing agents, and methyl bromide, an effective produce and soil fumigant,
contain bromine. All of these compounds have atmospheric lifetimes long enough to
allow them to be transported by winds into the stratosphere. The CFCs are so stable
that only one process breaks them down: exposure to strong UV radiation. When a CFC
molecule breaks down, it releases atomic chlorine which destroys ozone. One chlorine
atom can destroy over 100,000 ozone molecules. The net effect is to destroy ozone
faster than it is naturally created.

Figure 12 illustrates how the use of CFCs results in the destruction
of the ozone layer and the increase in UV radiation at the earthís surface.

Figure 12
Ozone Depletion Process

Refer to Section III for a more detailed discussion
of the sources of ozone depletion.